Surface acoustic wave resonator, surface acoustic wave oscillator, and surface acoustic wave module device

ABSTRACT

A surface acoustic wave resonator includes a piezoelectric substrate and an interdigital transducer (IDT) that includes electrode fingers exciting a surface acoustic wave on the piezoelectric substrate, a first region at a center of the IDT, and a second region and a third region at opposite sides of the IDT. In the IDT, a line occupation rate at which an electromechanical coupling coefficient becomes a maximum is different from the line occupation rate at which reflection of the surface acoustic wave becomes a maximum.

BACKGROUND

1. Technical Field

The present invention relates to a surface acoustic wave resonator usinga piezoelectric substrate, a surface acoustic wave oscillator, and asurface acoustic wave module device.

2. Related Art

Surface acoustic wave resonators using a surface acoustic wave (SAW)have been widely used as electronic apparatuses. In recent years, alongwith the popularization of portable apparatuses, it is required todownsize the surface acoustic wave resonators, which are used for theportable apparatuses. If the number of pairs of interdigital transducer(ITD) is reduced for downsizing the surface acoustic wave resonator, adecrease in a Q value or an increase in a crystal impedance (CI) valueis caused. Thereby, characteristics of the surface acoustic waveresonator cannot be substantially obtained. The decrease in the Q valueblocks excitation of a stable surface acoustic wave while the increasein the CI value causes an increase in power consumption in anoscillation circuit. As a countermeasure against these, for example, atechnique for downsizing a surface acoustic wave resonator is disclosedin JP-A-2004-194275. The surface acoustic wave resonator can bedownsized by dividing the IDT into three regions and electrode fingersin each region of the IDT are formed at a fixed pitch which differswithin 2%. As a result, the Q value is increased and the CI value isdecreased, allowing the surface acoustic wave resonator to be downsized.

However, as characteristics of a surface acoustic wave resonator, thelarger the Q value, the more excitation of a surface acoustic wave isstabilized. In addition, the smaller the CI value, the lower powerconsumption can be achieved. Thus, a surface acoustic wave is requiredthat is downsized and whose characteristics are enhanced.

SUMMARY

The invention is proposed in order to solve the above-mentioned problemsand can be achieved by the following aspects.

According to a first aspect of the invention, a surface acoustic waveresonator includes a piezoelectric substrate and an interdigitaltransducer (IDT) that includes electrode fingers exciting a surfaceacoustic wave on the piezoelectric substrate, a first region at a centerof the IDT, and a second region and a third region at opposite sides ofthe IDT. In the IDT, a line occupation rate at which anelectromechanical coupling coefficient becomes a maximum is differentfrom the line occupation rate at which reflection of the surfaceacoustic wave becomes a maximum. Each of the first, second, and thirdregions has a uniform electrode finger interval, and the electrodefinger intervals in the second and third regions are larger than theelectrode finger interval of the first region. The first region has afirst line occupation rate, the second region has a second lineoccupation rate, and the third regions has a third line occupation rate,and the first, second and third line occupation rates are uniform in thefirst, second, and third regions respectively, and an electromechanicalcoupling coefficient at the first line occupation rate is larger thanthe electromechanical coupling coefficients at the second lineoccupation rate and the third line occupation rate, and the reflectionof the surface acoustic wave at the second line occupation rate and thethird line occupation rate is larger than the reflection of the surfaceacoustic wave at the first line occupation rate. The line occupationrate is defined as a value obtained by dividing a width of the electrodefinger by an electrode finger interval that is an interval betweencenters of the adjacent electrode fingers.

According to the structure, the IDT, in which the line occupation rateat which the electromechanical coupling coefficient becomes the maximumis different from the line occupation rate at which the reflection of asurface acoustic wave becomes the maximum, is divided into threeregions, and the three regions are weighted by the electrode fingerinterval and the line occupation rate. Each of the first, second, andthird regions of the IDT has a uniform electrode finger interval, andthe electro finger intervals in the second and third regions are formedlarger than that in the first region. The first region has a first lineoccupation rate, the second region has a second line occupation rate,and the third regions has a third line occupation rate, and the first,second and third line occupation rates are uniform in the first, second,and third regions respectively, and an electromechanical couplingcoefficient at the first line occupation rate is larger than theelectromechanical coupling coefficients at the second line occupationrate and the third line occupation rate, and the reflection of thesurface acoustic wave at the second line occupation rate and the thirdline occupation rate is larger than the reflection of the surfaceacoustic wave at the first line occupation rate. A standing wave of asurface acoustic wave generated in the surface acoustic wave resonatorhas large oscillation displacement in the first region, which is thecenter of the IDT. On the other hand, the standing wave has smalloscillation displacement in the second and third regions, which areopposite sides of the IDT. In the first region having large oscillationdisplacement, a line occupation rate is selected at which theelectromechanical coupling coefficient becomes large in order tosuppress an increase in a CI value. On the other hand, in the second andthird regions having small oscillation displacement, a line occupationrate is selected at which reflection of a surface acoustic wave becomeslarge in order to enhance the oscillation energy confinement effect.Accordingly, the surface acoustic wave resonator having excellent CIvalue and Q value can be realized. As a result, the surface acousticwave resonator can be downsized while the Q value is improved, and thesurface acoustic wave resonator having low power consumption can beprovided by decreasing the CI value.

In the surface acoustic wave resonator, a ratio of an electrode fingerinterval PTc in the first region to an electrode finger interval PTs2 inthe second region is in a range of 1.006 or more to 1.014 or less, and aratio of the electrode finger interval PTc in the first region to anelectrode finger interval PTs3 in the third region preferably is in arange of 1.006 or more to 1.014 or less.

According to the structure, the surface acoustic wave resonator can beprovided whose Q value exceeds that of the related art surface acousticwave resonator of which the IDT is divided into three regions andelectrode fingers in each region of the IDT are formed at a fixed pitchwhich differs within 2%.

In the surface acoustic wave resonator, the first line occupation ratepreferably is a rate at which the electromechanical coupling coefficientbecomes the maximum, and the second line occupation rate and the thirdline occupation rate are rates at which the reflection of the surfaceacoustic wave becomes the maximum.

According to the structure, the surface acoustic resonator can beprovided whose Q value exceeds that of the related art surface acousticwave resonator.

In the surface acoustic wave resonator, the piezoelectric substratepreferably is a quartz substrate having an Euler angle of (−1° to +1°,113° to 135°, ±(40° to 49°)). In the resonator, a value of the firstline occupation rate preferably is in a range of 0.35 or more to 0.5 orless, and values of the second line occupation rate and the third lineoccupation rate preferably are less than 0.35.

According to the structure, the surface acoustic resonator can beprovided that exhibits excellent frequency temperature characteristicsand whose Q value is increased and whose CI value is reduced by using anin-plane rotated ST cut quartz substrate having the Euler angle of (−1°to +1°, 113° to 135°, ±(40° to 49°), setting a value of the first lineoccupation rate in a range of 0.35 or more to 0.5 or less, and settingvalues of the second line occupation rate and the third line occupationrate less than 0.35.

In the surface acoustic wave resonator, the piezoelectric substratepreferably is a quartz substrate having the Euler angle of (−1° to +1°,121° to 132°, −3° to +3°)). In the resonator, a value of the first lineoccupation rate preferably is in a range of 0.4 or more to 0.6 or less,and values of the second line occupation rate and the third lineoccupation preferably are more than 0.6.

According to the structure, the surface acoustic resonator can beprovided that exhibits excellent frequency temperature characteristicsand whose Q value is increased and whose CI value is reduced by using anin-plane rotated ST cut quartz substrate having the Euler angle of (−1°to +1°, 113° to 135°, ±(40° to 49°)), setting a value of the first lineoccupation rate in a range of 0.4 or more to 0.6 or less, and settingvalues of the second line occupation rate and the third line occupationrate more than 0.6.

According to a second aspect of the invention, a surface acoustic waveoscillator includes the surface acoustic wave resonator according to thefirst aspect and a circuit element. In the oscillator, the surfaceacoustic wave resonator and the circuit element are mounted in apackage.

According to the structure, since the surface acoustic wave resonatorwhose Q value is increased and whose CI value is decreased is mounted,an oscillation of a surface acoustic wave is stable. As a result, thesurface acoustic wave oscillator having low power consumption can beprovided.

According to a third aspect of the invention, a surface acoustic wavemodule device includes the surface acoustic wave resonator of the firstaspect mounted on a circuit substrate.

According to the structure, since the surface acoustic wave resonatorwhose Q value is increased and whose CI value is decreased is mounted,an oscillation of a surface acoustic wave is stable. As a result, thesurface acoustic wave oscillator having low power consumption can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a plan view schematically illustrating a structure of asurface acoustic wave resonator of a first embodiment.

FIG. 2 is a diagram illustrating a relationship between a position of anelectrode finger and an electrode finger interval of the firstembodiment.

FIG. 3 is a diagram illustrating a relationship between the position ofan electrode finger and a line occupation rate of the first embodiment.

FIG. 4 is a diagram illustrating a cutout angle a quartz substrate and asurface acoustic wave propagation direction thereof.

FIG. 5 is a graph illustrating a relationship between the lineoccupation rate and an effective coupling coefficient of an IDT of thefirst embodiment.

FIG. 6 is a graph illustrating a relationship between the lineoccupation rate and a Q value of the IDT of the first embodiment.

FIG. 7 is a graph illustrating a relationship between the lineoccupation rate and a frequency variation of the first embodiment.

FIG. 8 is a graph illustrating a relationship between a shift amount ofthe electrode finger interval and the Q value of the first embodiment.

FIG. 9 is a graph illustrating a relationship between the lineoccupation rate and the effective coupling coefficient of the IDT of asecond embodiment.

FIG. 10 is a graph illustrating a relationship between the lineoccupation rate and a normalized reflection amount of the secondembodiment.

FIG. 11 is a sectional view schematically showing a surface acousticwave oscillator of a third embodiment.

FIG. 12 is a circuit block diagram of a receiver module of a fourthembodiment.

FIG. 13 is a diagram illustrating a surface acoustic wave resonator of arelated art.

FIG. 14 is a diagram illustrating the line occupation rate.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings.

Comparative Example

First, a surface acoustic wave resonator of a related art will bedescribed for comparison with the embodiments of the invention. FIG. 13is a diagram illustrating a surface acoustic wave resonator based onJP-A-2004-194275. A surface acoustic wave resonator 100 includes aninterdigital transducer (IDT) 102 composed of interdigital electrodesand a pair of reflectors 103 formed in a manner sandwiching the IDT 102in a propagation direction of a surface acoustic wave. The IDT 102 andthe reflectors 103 are arranged on a quartz substrate 101 serving as apiezoelectric substrate.

The quartz substrate 101 is made of an in-plane rotated ST cut quartzsubstrate whose cut surface and whose surface acoustic wave propagationdirection are (−1° to +1°, 113° to 135°, ±(40° to 49°)) when they areexpressed by Euler angle (Φ, θ, Ψ). The IDT 102 is formed in a mannerthat electrode fingers 102 a and 102 b are alternately arranged so as tohave different polarities. Two electrode fingers, one of the electrodefingers 102 a and one of the electrode fingers 102 b, are referred to asa pair of electrode fingers. Further, the IDT 102 is divided into threeregions: a first region 104 a at the center of the IDT 102; and secondand third regions 104 b and 104 c at opposite sides thereof. Here, aninterval between the centers of the adjacent electrode fingers 102 a and102 b is denoted as an electrode finger interval PT. The electrodefinger interval PT is constant in each region. However, the electrodefinger intervals PT are different in the regions. A relationship ofPTc<PTs₂=PTs₃ is satisfied when the electrode finger interval of thefirst region 104 a is denoted as PTc, the electrode finger interval ofthe second region 104 b is denoted as PTs₂, and the electrode fingerinterval of the third region 104 c is denoted as PTs₃. The reflectors103 includes a multiple of electrode fingers 103 a aligned therein so asto be electrically neutral. In addition, a relationship ofPTc<PTs₂=PTs₃<PTr is satisfied when the electrode finger interval, aninterval between the centers of the adjacent electrode fingers 103 a, isdenoted as PTr.

A surface acoustic wave excited by the IDT 102 propagates along an arrowH, and proceeds to a direction orthogonal to the electrode fingers 102 aand 102 b. Here, a ratio of the electrode fingers to the IDT 102 and thereflectors 103 in the propagation direction of a surface acoustic waveis denoted as a line occupation rate η. Specifically, as FIG. 14 shows,PT=S+(L/2+L/2) and the line occupation rate η=(L/2+L/2)/PT are satisfiedwhen a width of the electrode fingers 102 a and 102 b is denoted as L, adimension of a space between the adjacent electrode fingers (a spacewhere no electrode fingers are formed) is denoted as S, and an intervalbetween the centers of the adjacent electrode fingers is denoted as PT.

The line occupation rate η is set to be constant (η=0.35) in the IDT 102and the reflectors 103. The IDT 102 and the reflectors 103 are formed ofaluminum (Al) and set to have a predetermined film thickness (0.06λ, λis a wave length of a surface acoustic wave). Further, in the IDT 102,there are 68 pairs of the electrode fingers in the first region 104 a,34 pairs in each of the second region 104 b and the third region 104 c,and 57 pairs in each reflector 103 (250 pairs in total). The respectiveelectrode finger intervals are PTc=10.00 μm, PTs=10.07 μm, and PTr=10.10μm. In the above surface acoustic wave resonator 100, characteristicsare realized as follow: a Q value is 20,500 and a CI value is 18Ω.

First Embodiment

Next, a first embodiment of the invention will be described. FIG. 1 is aplan view schematically illustrating a structure of the surface acousticwave resonator of the present embodiment. FIG. 2 is a diagramillustrating a relationship between a position of the electrode fingerand the electrode finger interval. FIG. 3 is a diagram illustrating arelationship between the position of the electrode finger and the lineoccupation rate. FIG. 4 is a diagram illustrating a cutout angle of aquartz substrate and a surface acoustic wave propagation directionthereof.

As FIG. 1 shows, a surface acoustic wave resonator 1 includes an IDT 12composed of interdigital electrodes and a pair of reflectors 13 formedin a manner sandwiching the IDT 12 in the propagation direction of asurface acoustic wave. The IDT 12 and the reflectors 13 are arranged ona quartz substrate 11 serving as a piezoelectric substrate. In thesurface acoustic wave resonator 1, a surface acoustic wave excited bythe IDT 12 propagates along the arrow H, and proceeds to a directionorthogonal to electrode fingers 12 a and 12 b.

The quartz substrate 11 is made of an in-plane rotated ST cut quartzsubstrate whose cut surface and whose surface acoustic wave propagationdirection are (−1° to +1°, 113° to 135°, ±(40° to 49°)) when they areexpressed by the Euler angle (φ, θ, Ψ). As FIG. 4 shows, crystal axes ofquartz crystal is defined by an X axis (electrical axis), a Y axis(mechanical axis), and a Z axis (optical axis). The Euler angle (0°, 0°,0°) of a quartz Z-plate 8 is perpendicular to the Z axis. Here, φ of theEuler angle (now shown) relates to the first rotation of the quartzZ-plate 8, and represents a first rotation angle when the Z axis is arotation axis and a direction to rotate from +X axis to +Y axis is apositive rotation angle. Further, θ of the Euler angle relates to thesecond rotation performed after the first rotation of the quartz Z-plate8, and represents a second rotation angle when the X axis after thefirst rotation is the rotation axis and a direction to rotate from +Yaxis after the first rotation to +Z axis is a positive rotation angle. Acut surface of the quartz substrate 11 is determined based on the firstrotation angle φ and the second rotation angle θ. Furthermore, Ψ of theEuler angle relates to the third rotation performed after the secondrotation of the quartz Z-plate 8, and represents a third rotation anglewhen the Z axis after the second rotation is the rotation axis and adirection to rotate from +X axis after the second rotation to +Y axisafter the second rotation is a positive rotation angle. The propagationdirection of a surface acoustic wave is represented by the thirdrotation angle w with respect to the X axis after the second rotation.For the surface acoustic wave resonator 1, the quartz substrate 11 isused whose first rotation angle φ is −1° to +1° and whose secondrotation angle θ is 113° to 135°. Additionally, the IDT 12 is arrangedsuch that the propagation direction of a surface acoustic wave is withina range of Ψ=±(40° to 49°). The angle Ψ is also referred to as anin-plane rotated angle. The in-plane rotated ST-cut quartz substrate hasa small frequency variation due to a change in temperature, exhibitingexcellent frequency temperature characteristics.

The IDT 12 is formed in a manner that the electrode fingers 12 a and 12b are alternately arranged so as to have different polarities. In thepresent embodiment, two electrode fingers, one of the electrode fingers12 a and one of the electrode fingers 12 b, are counted as a pair ofelectrode fingers. The IDT 12 is divided into three regions: a firstregion 14 a at the center of the IDT 12; and second and third regions 14b and 14 c at opposite sides thereof. Here, an interval between thecenters of the adjacent electrode fingers 12 a and 12 b is denoted asthe electrode finger interval PT. As FIG. 2 shows, the electrode fingerinterval PT is constant in each region. However, the electrode fingerintervals PT are different in the regions. A relationship ofPTc<PTs₂=PTs₃ is satisfied when the electrode finger interval of thefirst region 14 a is denoted as PTc, the electrode finger interval ofthe second region 14 b is denoted as PTs₂, and the electrode fingerinterval of the third region 14 c is denoted as PTs₃. Values of PTs₂ andPTs₃ may be different from each other as long as PTs₂ and PTs₃ aresmaller than a value of PTc.

The reflectors 13 includes a multiple of electrode fingers 13 a alignedtherein so as to be electrically neutral. The reflectors 13 may begrounded or coupled to either one of the electrode fingers 12 a and 12b. In addition, a relationship of PTc<PTs₂=PTs₃<PTr is satisfied whenthe electrode finger interval, an interval between the centers of theadjacent electrode fingers 13 a, is denoted as PTr. In the reflector 13,two adjacent electrode fingers 13 a are counted as a pair of electrodefingers.

A relationship between the line occupation rate and the position of theelectrode finger will be described. As FIG. 3 shows, the line occupationrate is constant in each region of the first region 14 a, the secondregion 14 b, and the third region 14 c of the IDT. However, the lineoccupation rates η are different in the regions. A relationship of ηc>ηsis satisfied when the line occupation rate of the first region 14 a isdenoted as ηc and the line occupation rates of the second and thirdregions 14 b and 14 c are denoted as ηs. Further, the line occupationrate is set to be ηc=ηr when the line occupation rate of the refractors13 is denoted as ηr.

The IDT 12 and the reflectors 13 are formed of aluminum (Al) and set tohave a predetermined film thickness (0.06λ, λ is a wave length of asurface acoustic wave). Further, in the IDT 12, there are 68 pairs ofthe electrode fingers in the first region 14 a, 34 pairs in each of thesecond region 14 b and the third region 14 c, and 57 pairs in eachreflector 103 (250 pairs in total). The respective electrode fingerintervals are PTc=10.00 μm, PTs=10.10 μm, and PTr=10.10 μm. Therespective line occupation rates are ηc=0.35, ηs=0.28, and ηr=0.35. Inthe above surface acoustic wave resonator 1, characteristics areobtained as follows: the Q value is 22,700 and the CI value is 16Ω. As aresult, compare with the above-described surface acoustic wave resonator100, the surface acoustic wave resonator 1 can be obtained whose Q valueand CI value are improved.

The structure of the IDT 12 will be described in detail from a designpoint of view. FIG. 5 is a graph illustrating a relationship between theline occupation rate of the IDT and an effective coupling coefficient.In the surface acoustic wave resonator, the CI value or an equivalentresistance R1 is important characteristics as well as the Q value. Bydecreasing the CI value or the equivalent resistance R1, the powerconsumption of the surface acoustic wave resonator can be suppressed. Inorder to decrease the CI value or the equivalent resistance R1, it isnecessary to increase an electromechanical coupling coefficient K². Theelectromechanical coupling coefficient K² can be obtained by thefollowing equation.K ²=(2(Vo−Vs)/Vo)×100[%]Here, Vo is a propagation velocity of a surface acoustic wave when eachelectrode finger of the IDT is in an electrically open state while Vs isthe propagation velocity of a surface acoustic wave when each electrodefinger of the IDT is in an electrically short-circuit state. Theelectromechanical coupling coefficient when the line occupation rate ηis varied is obtained by a difference between the normalized velocitiesof the opened state and the short-circuit state of the IDT, andindicated as an effective coupling coefficient in FIG. 5.

As FIG. 5 shows, as the line occupation rate η becomes larger than 0.2,the effective coupling coefficient increases. When the line occupationrate η is approximately 0.4, the effective coupling coefficient becomesthe maximum. The effective coupling coefficient at this time isapproximately 0.065%, and the CI value or the equivalent resistance R1becomes the minimum. Then, the line occupation rate η becomes largerthan 0.4, the effective coupling coefficient decreases. In the presentembodiment, a value in a range of 0.35 or more to 0.5 or less isselected for the first region of the IDT 12 as a line occupation rate ηat which the effective coupling coefficient is large.

FIG. 6 is a graph illustrating a relationship between the lineoccupation rate and the Q value. As the line occupation rate η becomeslarger than 0.15, the Q value increases. When the line occupation rate ηis approximately 0.2, the Q value becomes the maximum. The Q value atthis time is approximately 20,000. Then, as the line occupation rate ηbecomes larger than 0.2, the Q value decreases. The Q value andreflection amount are correlative. Accordingly, when the Q value is themaximum, the reflection amount is substantially the maximum as well.That is, the reflection of a surface acoustic wave is substantially themaximum when the line occupation rate η is approximately 0.2. At thistime, the resonator exhibits excellent oscillation energy confinement,thereby the Q value becomes the maximum. Therefore, in the presentembodiment, values less than 0.35 are selected for the second and thirdregions of the IDT 12 as a line occupation rate η at which reflection islarge.

As shown in FIGS. 5 and 6, in the IDT of the present embodiment, theline occupation rate at which the electromechanical coupling coefficientbecomes the maximum is different from the line occupation rate at whichthe Q value and the reflection of a surface acoustic wave becomes themaximum. In the present embodiment, the IDT is divided into threeregions, and the three regions are weighted by the electrode fingerinterval and the line occupation rate. In the first region of the IDT12, a value in a range of 0.35 or more to 0.5 or less is set as a lineoccupation rate at which the effective coupling coefficient is large. Inthe second and third regions, which are the end portions of the IDT 12,values less than 0.35 are set as a line occupation rate so as to havelarge reflection. A standing wave of a surface acoustic wave generatedin the surface acoustic wave resonator 1 has large oscillationdisplacement in the first region 14 a arranged at the center of the IDT12. On the other hand, the standing wave has small oscillationdisplacement in the second and third regions 14 b and 14 c arranged atthe opposite sides thereof. In the first region 14 a having largeoscillation displacement, a line occupation rate is selected at whichthe electromechanical coupling coefficient is large in order to suppressan increase in the CI value. On the other hand, in the second and thirdregions 14 b and 14 c having small oscillation displacement, a lineoccupation rate is selected at which reflection of a surface acousticwave is larger in order to enhance the oscillation energy confinementeffect. Accordingly, the surface acoustic wave resonator 1 havingexcellent CI value and Q value can be realized. As a result, the surfaceacoustic wave resonator 1 can be downsized while the Q value isimproved, and the surface acoustic wave resonator 1 having low powerconsumption can be provided by decreasing the CI value.

FIG. 7 is a graph illustrating a relationship between the lineoccupation rate of the IDT and a frequency variation. As FIG. 7 shows,as the line occupation rate η becomes larger than 0.1, the frequencyvariation decreases. When the line occupation rate η is approximately0.4, the frequency variation becomes the minimum.

The first region 14 a of the IDT 12 having large oscillationdisplacement has higher sensitivity of frequency variation when the lineoccupation rate η varies. The frequency greatly varies in accordancewith the variation of the line occupation rate η. On the other hand, thesecond and third regions 14 b and 14 c have a small frequency variation.Therefore, even if the line occupation rate η varies in these regions,there is only small effect on the frequency of the surface acoustic waveresonator 1. Accordingly, if a range of a line occupation rate η havinga small frequency variation is selected for the first region 14 a, thefrequency variation can be reduced when the line occupation rate ηvaries. In the present embodiment, a value in a range of 0.35 or more to0.5 or less is selected as the line occupation rate ηc of the firstregion 14 a. As a result, variations in frequency during manufacture canbe reduced, being able to obtain the surface acoustic wave resonator 1having high frequency accuracy.

FIG. 8 is a graph illustrating a relationship between a shift amount ofthe electrode finger interval and the Q value. The shift amount of theelectrode finger interval is a ratio of PTs/PTc. Here, PTc is theelectrode finger interval of the first region 14 a while PTs is theelectrode finger interval of the second region 14 b or that of the thirdregion 14 b. The related art surface acoustic wave resonator 100 obtainsthe maximum Q value 20,500 when the shift amount of the electrode fingerinterval is 1.007. The surface acoustic wave resonator 1 of the presentembodiment obtains the maximum Q value 22,700 when the shift amount ofthe electrode finger interval is 1.010, being able to increase the Qvalue of about 11% comparing with the related art surface acoustic waveresonator 100. By setting the shift amount of the electrode fingerinterval in a range of 1.006 or more to 1.014 or less, the surfaceacoustic wave resonator 1 can be obtained whose Q value is increasedcomparing with the related art surface acoustic wave resonator 100.

Second Embodiment

A surface acoustic wave resonator of a second embodiment will bedescribed. In the present embodiment, the surface acoustic wavepropagation direction of the quartz substrate and values of the lineoccupation rate thereof are different from those in the firstembodiment. The quartz substrate is made of an in-plane rotated ST cutquartz substrate whose cut surface and whose surface acoustic wavepropagation direction are (−1° to +1°, 113° to 135°, −3° to 3°) whenthey are expressed by the Euler angle (φ, θ, Ψ). In the surface acousticwave resonator, the IDT is arranged in a manner that the propagationdirection of a surface acoustic wave is in the X axis (Ψ=0°±3°).Further, the electrode fingers intervals in each region of the IDT isthe same as those in the first embodiment (refer to FIGS. 1 and 2), anddescriptions of the structure of the surface acoustic wave resonatorwill be omitted.

FIG. 9 is a graph illustrating a relationship between the lineoccupation rate and effective coupling coefficient of the IDT. FIG. 10is a graph illustrating a relationship between the line occupation rateand a normalized reflection amount. These graphs illustrate therelationship with the line occupation rate when a quartz substrate ofthe Euler angle (0°, 123°, 0°) is used as an example of a quartzsubstrate where a thickness of an aluminum electrode is 0.04λ. As FIG. 9shows, as the line occupation rate η becomes larger than 0.2, theeffective coupling coefficient increases. When the line occupation rateη is approximately 0.5, the effective coupling coefficient becomes themaximum. The effective coupling coefficient at this time isapproximately 0.075%, and the CI value or the equivalent resistance R1is the minimum under the same Q value. Then, as the line occupation rateη becomes larger than 0.5, the effective coupling coefficient decreases.In the present embodiment, a value in a range of 0.4 or more to 0.6 orless is selected for the first region of the IDT 12 as a line occupationrate at which the effective coupling coefficient is large.

As FIG. 10 shows, as the line occupation rate η becomes larger than 0.2,the normalized reflection amount increases. Here, the normalizedreflection amount is a reflection amount of a surface acoustic waveobtained by a decreasing rate of frequency generated by reflection. Tobe specific, the normalized reflection amount is a parameter expressedas [(Fh−F1)/2]/(Vf/λ) when a resonant frequency at a lower end of a stopband is F1, a resonant frequency at an upper end of the stop band is Fh,a surface acoustic wave propagation velocity at a free surface of thequartz substrate 11 is Vf, and a wavelength of a surface acoustic waveis λ. The reflection amount becomes substantially the maximum when theline occupation rate η is approximately 0.8. At this time, the resonatorexhibits excellent oscillation energy confinement, thereby the Q valuebecomes the maximum. Accordingly, in the present embodiment, values morethan 0.6 are selected for the second and third regions of the IDT 12 asa line occupation rate η at which the reflection is large. In the IDT 12of the present embodiment, a line occupation rate of a value in a rangeof 0.35 or more to 0.5 or less, in which the effective couplingcoefficient is large, is selected for the first region while a lineoccupation rate of a value more than 0.6, in which the reflection islarge, is selected for the second and third regions, which are the endportions of the IDT 12.

A standing wave of a surface acoustic wave generated in the surfaceacoustic wave resonator has large oscillation displacement in the firstregion, which is the center of the IDT. On the other hand, the standingwave has small oscillation displacement in the second and third regions,which are the opposite sides of the IDT. In the first region havinglarge oscillation displacement, a line occupation rate is selected atwhich the electromechanical coupling coefficient is large in order tosuppress an increase in the CI value. On the other hand, in the secondand third regions having small oscillation displacement, a lineoccupation rate is selected at which the reflection of a surfaceacoustic wave is large in order to enhance the oscillation energyconfinement effect. Accordingly, the surface acoustic wave resonator 1can be downsized while the Q value is improved, and the surface acousticwave resonator 1 having low power consumption can be provided bydecreasing the CI value.

In the first and second embodiments, aluminum is used for an electrodematerial of the IDT and the reflectors. However, the same effect isachieved by an aluminum alloy. Except for aluminum, gold (Au), silver(Ag), copper (Cu), tungsten (W), tantalum (Ta) or an alloy mainlycontaining any of the above may be used as the electrode material. Thefilm thicknesses of the electrode of the IDT in the first and secondembodiments are respectively 0.06λ and 0.04λ (λ is a wavelength of asurface acoustic wave). However, it is confirmed that the same effect isobtained with other film thickness. Further, in the first and secondembodiments, the line occupation rate ηc of the first region of the IDTand the line occupation rate nr of the reflectors are set to be equal.However, ηc and ηr are not necessarily set to be equal. For example, theline occupation rate ηr may be set so as to increase reflection at thereflectors. In this case, the line occupation rate ηr may be set to beless than 0.35 in the first embodiment, and that may be set to be morethan 0.6 in the second embodiment. The line occupation rate of thereflectors may be set to be equal to that of at least one of the secondregion and the third region. Alternatively, the first to third regionsof the IDT and the reflectors may respectively have different lineoccupation rates. Further, in the first and second embodiments, thesecond and third regions have the same electrode finger interval.However, the second and third regions may have different electrodefinger intervals from each other. Though the reflectors are arranged atthe opposite sides of the IDT in the first and second embodiments, thesame effect can be obtained without the reflectors.

Third Embodiment

By mounting the surface acoustic wave resonator of any of the embodimentin a package, a surface acoustic wave oscillator can be configured. FIG.11 is a sectional view schematically illustrating a surface acousticwave oscillator including the surface acoustic wave resonator mounted ina package. A surface acoustic wave oscillator 30 includes a ceramicpackage 31, an IC chip 32, the surface acoustic wave resonator 1, a lid37, and the like. Formed in the ceramic package 31 is a recess 38 thatis opened. Formed in the ceramic package 31 is a seam ring 35 in amanner surrounding the recess 38. The seam ring 35 is formed of a metalmaterial such as kovar, In addition, on the periphery surface of theceramic package 31, an external connection electrode 36 is formed tomake connection with the external such as a circuit substrate. Though itis not illustrated in the drawing, wiring lines are provided so as tocouple the external connection electrode 36 to the inside of the recess38 of the ceramic package 31.

The IC chip 32 is fixed to the bottom surface of the recess 38, and ismounted by a metal wire such as a gold wire. The IC chip 32 is providedwith an oscillation circuit exciting the surface acoustic wave resonator1. The IC chip 32 may include a temperature compensation circuit, avoltage control circuit, and the like. The surface acoustic waveresonator 1 is fixed to a shelf of the recess 38 with an adhesive 34. Apad to be coupled to the IDT and the wiring lines in the ceramic package31 are coupled by metal wires 33.

On the upper side of the recess 38, the lid 37 made of a metal material,such as kovar, is arranged and air-tightly seals the recess 38 by seamwelding the lid 37 and the seam ring 35. Since the surface acoustic waveresonator 1 whose Q value is increased and whose CI value is decreasedis mounted in the ceramic package 31, an oscillation of a surfaceacoustic wave is stable. As a result, the surface acoustic waveoscillator 30 consuming lower power can be obtained.

Fourth Embodiment

By mounting the surface acoustic wave resonator of any of theembodiments, a surface acoustic wave module device can be configured.FIG. 12 is a circuit block diagram of a receiver module configured bymounting the surface acoustic wave resonator on a circuit substrate asan example of the surface acoustic wave module device.

A receiver module 40 includes a receiving antenna 41, a low-noiseamplifier (LNA) 42, a mixer 43, a local oscillator 44, an intermediatefrequency (IF) amplifier 45, and a detector 46. The receiving antenna 41is coupled to an input of the mixer 43 through the LNA 42. The localoscillator 44 is also coupled to the input of the mixer 43. The localoscillator 44 includes the surface acoustic wave resonator and anoscillation circuit exciting the surface acoustic wave resonator.Accordingly, the local oscillator 44 can reliably output frequencysignals to the mixer 43. The IF amplifier 45 and the detector 46 arecoupled to an output of the mixer 43 in series.

A signal transmitted from a transmitter, which is the other side, isinputted the LNA 42 through the receiving antenna 41. After beingamplified in the LNA 42, the signal is inputted to the mixer 43. Afrequency signal from the local oscillator 44 is inputted to the mixer43. The mixer 43 down-converts the signal inputted from the LNA 42 andoutputs the signal. The down-converted signal is amplified in the IFamplifier 45 and inputted to the detector 46 so as to be detected.Accordingly, the receiver module 40 can receive signals transmitted fromthe transmitter. Since the receiver module 40 includes the surfaceacoustic wave resonator of any of the embodiments in the localoscillator 44, the receiver module 40 can stably receive signals. As aresult, the receiver module consuming low power can be obtained. Inaddition, the above receiver module can be provided to an exterior orthe like so as to be functioned as an electronic apparatus.

The entire disclosure of Japanese Patent Application No. 2008-273971,filed Oct. 24, 2008 is expressly incorporated by reference herein.

1. A surface acoustic wave resonator, comprising: a piezoelectricsubstrate; and an interdigital transducer (IDT) that includes electrodefingers exciting a surface acoustic wave on the piezoelectric substrate,a first region at a center of the IDT, and a second region and a thirdregion at opposite sides of the IDT, wherein in the IDT, a lineoccupation rate at which an electromechanical coupling coefficientbecomes a maximum is different from the line occupation rate at whichreflection of the surface acoustic wave becomes a maximum, wherein eachof the first, second, and third regions has a uniform electrode fingerinterval, and the electrode finger intervals in the second and thirdregions are larger than the electrode finger interval of the firstregion, wherein the first region has a first line occupation rate, thesecond region has a second line occupation rate, and the third regionshas a third line occupation rate, and the first, second and third lineoccupation rates are uniform in the first, second, and third regionsrespectively, and an electromechanical coupling coefficient at the firstline occupation rate is larger than the electromechanical couplingcoefficients at the second line occupation rate and the third lineoccupation rate, and the reflection of the surface acoustic wave at thesecond line occupation rate and the third line occupation rate is largerthan the reflection of the surface acoustic wave at the first lineoccupation rate, and wherein the line occupation rate is defined as avalue obtained by dividing a width of the electrode finger by anelectrode finger interval that is an interval between centers of theadjacent electrode fingers.
 2. The surface acoustic wave resonatoraccording to claim 1, wherein a ratio of an electrode finger intervalPTc in the first region to an electrode finger interval PTs₂ in thesecond region is in a range of 1.006 or more to 1.014 or less, and aratio of the electrode finger interval PTc in the first region to anelectrode finger interval PTs₃ in the third region is in a range of1.006 or more to 1.014 or less.
 3. The surface acoustic wave resonatoraccording to claim 1, wherein the first line occupation rate is a rateat which the electromechanical coupling coefficient becomes the maximum,and the second line occupation rate and the third line occupation rateare rates at which the reflection of the surface acoustic wave becomesthe maximum.
 4. The surface acoustic wave resonator according to claim1, wherein the piezoelectric substrate is a quartz substrate having anEuler angle of (1° to +1°, 113° to 135°, ±(40° to 49°)), wherein a valueof the first line occupation rate is in a range of 0.35 or more to 0.5or less, and values of the second line occupation rate and the thirdline occupation rate are less than 0.35.
 5. The surface acoustic waveresonator according to claim 1, wherein the piezoelectric substrate is aquartz substrate having the Euler angle of (1° to +1°, 121° to 132°, −3°to +3°)), wherein a value of the first line occupation rate is in arange of 0.4 or more to 0.6 or less, and values of the second lineoccupation rate and the third line occupation rate are more than 0.6. 6.A surface acoustic wave oscillator, comprising: the surface acousticwave resonator according to claim 1; and a circuit element, wherein thesurface acoustic wave resonator and the circuit element are mounted in apackage.
 7. A surface acoustic wave module device, comprising: thesurface acoustic wave resonator according to claim 1 mounted on acircuit substrate.